Transmitter noise in system budget

ABSTRACT

One embodiment provides an apparatus. The example apparatus includes a root mean square (RMS) distortion determination module configured to determine an RMS distortion error and a signal to noise and distortion ratio (SNDR), the RMS distortion error determined based, at least in part, on a portion of a transmitted pulse centered at or near a transmitted pulse maximum amplitude and the SNDR determined based, at least in part, on the RMS distortion error.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a U.S. Continuation Application of U.S. applicationSer. No. 14/142,308 filed Dec. 27, 2013, and claims the benefit of U.S.Provisional Application No. 61/845,698 filed Jul. 12, 2013, both ofwhich are incorporated herein by reference in their entirety.

FIELD

The present disclosure relates to transmitter noise, and, moreparticularly, to transmitter noise in a system budget.

BACKGROUND

A system budget in a communication system may specify allowableperformance characteristics, e.g., jitter, noise, distortion, etc., forthe communication system. The communication system typically includes atransmitting device, a communications link (that may include one or morechannels) and a receiving device. A portion of the system budget isallocated to each component. Testing is typically performed on eachcomponent of the communication system individually to determine and/orconfirm that the component operates within its respective portion of thesystem budget. In this testing, performance characteristics are assumedfor upstream components when testing downstream components. For example,conventional channel testing typically assumes a transmitter with aspecified jitter characteristic. Actual transmitter impairments mayinclude jitter, noise and/or distortion. Thus, such testing mayover-estimate or under-estimate actual transmitter performancecharacteristics when determining the system budget.

BRIEF DESCRIPTION OF DRAWINGS

Features and advantages of the claimed subject matter will be apparentfrom the following detailed description of embodiments consistenttherewith, which description should be considered with reference to theaccompanying drawings, wherein:

FIG. 1 illustrates a functional block diagram of a system consistentwith various embodiments of the present disclosure;

FIG. 2 illustrates a plot of an example test pattern that includes fourstatic levels consistent with various embodiments of the presentdisclosure;

FIG. 3 is a flowchart of testing operations according to variousembodiments of the present disclosure;

FIG. 4 is a flowchart of performance parameter determination operationsaccording to various embodiments of the present disclosure; and

FIG. 5 is another flowchart of performance parameter determinationoperations according to various embodiments of the present disclosure.

Although the following Detailed Description will proceed with referencebeing made to illustrative embodiments, many alternatives,modifications, and variations thereof will be apparent to those skilledin the art.

DETAILED DESCRIPTION

Generally, this disclosure relates to accounting for transmitter noisein a system budget. The methods and systems are configured to distributethe system budget between a transmitter, a communication channel and areceiver in a communication system. System budgets may specifytransmitter performance parameters (e.g., including jitter, a signal tonoise and distortion ratio (SNDR), a linear fit error and far-endnoise), a channel operation margin (COM) that is related to thetransmitter performance parameters and a receiver tolerance that isrelated to the transmitter performance parameters and the channeloperating margin. For a transmitter, conventional specifications set aminimum SNDR that is measured across all phases of a pulse response unitinterval (UI) and a linear fit error that is averaged across all phasesof the pulse response UI. For the channel operating margin, conventionalspecifications include a parameter related to transmitter jitter. For areceiver, conventional specifications calibrate transmitter noise andthen a target COM.

In an embodiment, methods and systems consistent with the presentdisclosure are configured to determine an SNDR based, at least in parton, a root mean square (RMS) distortion error and to determine the RMSdistortion error over a portion of the pulse response UI that may becentered at or near a peak of the pulse. For a pulse amplitude modulatedsignal, with more than two levels, i.e., PAM-x, x>2, methods and systemsconsistent with the present disclosure are configured to determinestatic levels and to determine a linear fit and the SNDR based, at leastin part, on the determined static levels. Methods and systems consistentwith the present disclosure are configured to include a Gaussian,signal-dependent noise term representing transmitter noise in thedetermination of COM. For PAM-x (x>2) modulation, COM may be determinedbased, at least in part, on a signal degradation factor that is based,at least in part, on the determined static levels. Thus, the systembudget may be generally more evenly allocated across system componentsand transmitter performance requirements may more accurately reflectactual transmitter performance characteristics.

FIG. 1 illustrates a functional block diagram of an example system 100according to various embodiments of the present disclosure. The system100 includes a node 102. System 100 may include a measurement device 104and a communications link 106, e.g., during testing operations, asdescribed herein. The system 100 may include a link partner 105 and acommunications link 107, e.g., during normal operation. Measurementdevice 104 may be directly coupled to node 102 (e.g., for transmitterperformance measurements) and/or may be coupled to node 102 viacommunication link 106 (e.g., for channel performance measurements).

The node 102 may be configured to communicate with link partner 105and/or measurement device 104, via, e.g., a respective communicationlink 107, 106, using a switched fabric communications protocol, forexample, an Ethernet communications protocol. The Ethernetcommunications protocol may be capable of providing communication usinga Transmission Control Protocol/Internet Protocol (TCP/IP). The Ethernetprotocol may comply or be compatible with the Ethernet standardpublished by the Institute of Electrical and Electronics Engineers(IEEE) titled “IEEE 802.3 Standard”, published in March, 2002 and/orlater versions of this standard, for example, the IEEE 802.3 Standardfor Ethernet, published 2012. The Ethernet protocol may comply or becompatible with the Ethernet standard published by the IEEE titled “IEEEP802.3bj™/D3.0 Draft Standard for Ethernet Amendment X: Physical LayerSpecifications and Management Parameters for 100 Gb/s Operation OverBackplanes and Copper Cables”, published Nov. 18, 2013, and/or earlierand/or later versions of this standard. Of course, in other embodiments,the switched fabric communications protocol may include a custom and/orproprietary switched fabric communications protocol.

“Node” may represent a computer node element (e.g., host server system,desktop computer, laptop computer, tablet computer, etc.), switch,router, hub, network storage device, network attached device,non-volatile memory (NVM) storage device, cloud-based server and/orstorage system, a blade, a smartphone, etc. The node 102 includes anetwork interface 114 (e.g., network interface card, etc.), a systemprocessor 110 (e.g., multi-core general purpose processor, such as thoseprovided by Intel® Corp., etc.) and system memory 112. The measurementdevice 104 may include a processor 130, memory 132 and a networkinterface 134 similar to network interface 114.

The network interfaces 114, 134 include respective PHY circuitry 116,136 generally configured to interface the node 102 with the measurementdevice 104. PHY circuitry 116 may be further configured to interface thenode 102 with link partner 105 via a communications link, e.g.,communications link 107. For example, node 102 may be coupled to linkpartner 105 during normal operation and may be coupled to measurementdevice 104 during testing, as described herein. PHY circuitry 116, 136may comply or be compatible with, the aforementioned switched fabriccommunications protocols, which may include, for example, 10GBASE-KR,40GBASE-KR4, 40GBASE-CR4, 100GBASE-CR10, 100GBASE-CR4, 100GBASE-KR4,and/or 100GBASE-KP4 and/or other PHY circuitry that is compliant withanother and/or after-developed communications protocol.

PHY circuitry 116, 136 includes respective transmit circuitry (Tx) 118,138 configured to transmit test signals from the node 102 to themeasurement device 104 and commands and/or data, e.g., test parameters,from the measurement device 104 to the node 102, respectively. PHYcircuitry 116, 136 further includes respective receive circuitry (Rx)120, 140 configured to receive commands and/or data from the measurementdevice 104 and test signals from the node 102, respectively. Of course,PHY circuitry 116,136 may also include encoding/decoding circuitryand/or equalization circuitry (not shown) configured to performanalog-to-digital and digital-to-analog conversion, encoding anddecoding of data, analog parasitic cancellation (for example, cross talkcancellation), equalization, and recovery of received data. Thecommunications link 106 may comprise, for example, a media dependentinterface that may include, for example, copper twin-axial cable,backplane traces on a printed circuit board, etc.

Transmit circuitry (Tx) 118 may be configured to transmit test signals,data packets and/or frames from the node 102 to the link partner 105,via communications link 107, and receive circuitry (Rx) 120 configuredto receive data packets and/or frames from the link partner 105, vialink 107. The communications link 107 may comprise, for example, a mediadependent interface that may include, for example, copper twin-axialcable, backplane traces on a printed circuit board, etc. In someembodiments, the communications link 107 may include a plurality oflogical and/or physical channels (e.g., differential pair channels) thatprovide separate connections between, for example, the Tx and Rx 118/120of the node 102 and receive and transmit circuitry of the link partner105. The link partner 105 may be configured to tolerate transmitter 118performance characteristics, determined as described herein.

Measurement device 104 may include an RMS distortion module 142, astatic level determination module 144 and a COM determination module146. Modules 142, 144, 146 may be included in test system 148.Measurement device 104 may further include a system budget test module150 configured to manage test system 148. Node 102 may include testsignal source 122 that may be included in network interface 114. In someembodiments, node 102 may include test system 124 that may include oneor more modules corresponding to modules 142, 144, 146.

Node 102 may be coupled to measurement device 104 in order to measureperformance characteristics of node 102 and, in particular, Tx 118. Insome embodiments, node 102 may include test point(s) TP 119 configuredto couple, e.g., measurement device 104, to Tx 118 to facilitatemeasurements. In some embodiments, node 102 may be coupled tomeasurement device 104 by communications link 106.

Test signal source 122 is configured to generate and provide a selectedtest signal that may be utilized for a performance parametermeasurement. The test signal may include one or more pulses that may bemodulated. For example, the test signal may include a single pulse ofduration one unit interval (UI). In this example, the single pulse maybe utilized to determine a pulse response. In another example, the testsignal may include a pulse stream modulated by a test bit sequence.Modulation may include pulse amplitude modulation (PAM), non-return tozero (NRZ), etc. PAM-x may include a number x levels (i.e., pulseamplitudes) where x>2. For example, PAM-4 corresponds to pulse amplitudemodulation with four levels, thus a PAM-4 pulse may carry two bits ofinformation. The test bit sequence may include a predefined bit sequencethat is known by system budget test module 150. For example, the testbit sequence may correspond to a pseudo-random bit sequence (PRBS). Inanother example, the test bit sequence may include a test pattern. Thetest pattern may include a bit sequence that is repeated each intervalof a plurality of intervals. The test pattern may be utilized todetermine a plurality of static levels corresponding to a number oflevels x for PAM-x.

FIG. 2 illustrates a plot 200 of an example test pattern 202 thatincludes four static levels 206, 208, 210, 212. Test pattern 202 may beutilized as a test signal for PAM-4. In the plot 200, the horizontalaxis corresponds to time and is measured in unit intervals (UIs) and thevertical axis corresponds to voltage (i.e., pulse amplitude). In thetest pattern 202, the test signal is configured to dwell at each level206, 208, 210, 212 for a time duration of level period 204. In thisexample, the level period duration 204 is 16 UIs. This level periodduration 204 is configured to reduce effects of inter-symbolinterference (ISI).

System budget test module 150 may be configured to select a test signalbased, at least in part, on the performance parameter being evaluatedand to provide a request to node 102. The Tx 118 is then configured totransmit the selected test signal. System budget test module 150 maythen be configured to capture the transmitted test signal. For example,measurement device 104 may be coupled to TP 119. In another example,measurement device 104 may be coupled to node 102 via communication link106.

Static level determination module 144 is configured to determine staticlevels based, at least in part, on the captured test signal. Staticlevels correspond to the voltages associated with each level of a PAMsignal. The transmitted test signal may correspond to a test pattern,e.g., test pattern 202. Ideally, in pulse amplitude modulation, eachlevel is equidistant from each adjacent level, i.e., uniform levelspacing. In other words, a voltage differential (“level differential”)between adjacent levels is the same for each pair of adjacent levels.Such uniformity is configured to facilitate detection of each level. Forexample, for PAM-4 modulation with a normalized pulse amplitude between+1 and −1, the four levels may correspond to −1, −⅓, ⅓ and 1. Inactuality, the level spacing may not be uniform and a size of thevariation between level differentials is related to a transmitterperformance characteristic.

Static level determination module 144 is configured to determine thetransmitted amplitude of each level. For test pattern 202, the staticlevels 206, 208, 210, 212 are related to voltages V_(A), V_(B), V_(C)and V_(D), respectively. Voltages V_(A), V_(B), V_(C) and V_(D) may bemeasured at or near a center of each level period 204 of static levels206, 208, 210, 212. For example, for a static level period duration of16 UI, V_(A), V_(B), V_(C) and V_(D) may be measured over two UIs, e.g.,from 7 UIs to 9 UIs for each level 206, 208, 210, 212. Of course, moreor fewer static levels (i.e., x) may similarly be determined for PAMmodulation with more or fewer static levels (i.e., PAM-x).

Static level determination module 144 may then be configured todetermine a minimum eye amplitude (i.e., a minimum static leveldifferential), a level mismatch ratio (i.e., a signal degradationfactor), a DC offset and at least one normalized effective mid-levelvoltage. For example, for a PAM-4 test signal, static leveldetermination module 144 may be configured to determine a minimum staticlevel differential, S_(min), as:

${S_{m\; i\; n} = \frac{\min\left( {{V_{D} - V_{C}},{V_{C} - V_{B}},{V_{B} - V_{A}}} \right)}{2}},$a level mismatch ratio, R_(LM), as:

${R_{LM} = \frac{6*S_{m\; i\; n}}{V_{D} - V_{A}}},$a DC offset, V_(Avg), as:

${V_{Avg} = \frac{V_{A} + V_{B} + V_{C} + V_{D}}{4}},$and normalized effective mid-level voltages, V₁ and V₂ as:

${V_{1} = \frac{V_{B} - V_{Avg}}{V_{A} - V_{Avg}}};{V_{2} = {\frac{V_{C} - V_{Avg}}{V_{D} - V_{Avg}}.}}$The factor six in the numerator of R_(LM) is related to the number oflevel differentials in the transmitted signal, e.g., three for PAM-4.For example, there are six (i.e., 2*3) level differential halves in apeak-to-peak PAM-4 signal.

The minimum eye amplitude, S_(min), and normalized mid-level voltages,V₁ and V₂, may be utilized by RMS distortion determination module 142and/or COM determination module 146, as described herein. The levelmismatch ratio, R_(LM), corresponds to a signal degradation factor andis a transmitter performance characteristic that may be included in afigure of merit determination, as described herein. In an embodiment,R_(LM) greater than or equal to 0.91 may be considered compliant. Forexample, a measured R_(LM) equal to 0.92 may be considered compliant. Inanother example, a measured R_(LM) equal to one may be consideredcompliant. R_(LM) equal to one corresponds to no level mismatch, i.e.,uniform static level differentials.

RMS distortion determination module 142 is configured to determine RMSdistortion based, at least in part, on a captured test signal. Thetransmitted test signal may be a pattern configured to enablemeasurement and/or determination of a response to a single pulse ofduration one unit interval (UI), i.e., a pulse response, and the RMSdistortion module 142 may then be configured to determine the pulseresponse based, at least in part, on the captured test signal. The pulseresponse may include M samples (i.e., phases) per UI. RMS distortiondetermination module 142 may be configured to determine a linear fit(using, e.g., a linear regression) to the pulse response, p(k) where k=1, . . . ,M is the phase index. Linear fit may be utilized for noisymeasurements to extract a linear part of a response from the noisymeasurements when the transmitted bit sequence is known. The remainder(i.e., the part of the measurement that is not linear) may then beconsidered noise and may include measurement and/or transmitter noise.In conventional systems, the linear fit may be determined utilizinguniform static levels, e.g., −1, −⅓, ⅓ and 1 for PAM−4 modulation.Consistent with the present disclosure, linear fit may be determinedutilizing normalized effective mid-levels (e.g., −1, −V₁, V₂, 1 forPAM-4) that may not be uniform. The normalized effective mid-levels(i.e., mid-level voltages) may be determined by static leveldetermination module 144, as described herein. The standard deviation ofthe error between the measured pulse response and the linear fit to themeasured pulse response may then be independent of level mismatch. Inconventional systems, linear fit may be determined based, at least inpart, on a plurality of measurements that are averaged prior to thelinear fit determination. Linear fit, consistent with the presentdisclosure may be determined based, at least in part, on the pulseresponse (i.e., that has not been averaged).

RMS distortion determination module 142 may then be configured todetermine a phase index, k_(p), that corresponds to a maximum value ofthe pulse response, i.e., k_(p)=arg (max p(k)), k=1, . . . , M. RMSdistortion determination module 142 may then be configured to determine,σ_(e)(k), the standard deviation of the error between the measured pulseresponse and the linear fit to the measured pulse response. An RMS (rootmean square) distortion error, i.e., a maximum standard deviation of theerror between the measured pulse response and the linear fit to themeasured pulse response, “max σ_(e)(k)”, may then be determined by theRMS distortion determination module 142. In conventional systems, theRMS distortion error may be determined over a full pulse UI. An RMSdistortion error, consistent with the present disclosure, may bedetermined over a portion of the pulse UI. The portion of the pulse UImay be centered at or near phase index k_(p) corresponding to themaximum value of the pulse response p(k). In an embodiment, the portionof the pulse UI may correspond to one half of the pulse UI, e.g.,

$k \in {\left\lbrack {{k_{p} - \frac{M}{4}},{k_{p} + \frac{M}{4}}} \right\rbrack.}$Determining RMS distortion error over a portion of the pulse UI mayreduce effects of jitter in determining the RMS distortion error. TheRMS distortion determination module 142 may then be configured todetermine a signal to noise plus distortion ratio, SNDR, as:

${SNDR} = {{20\;\log_{10}\frac{S_{m\; i\; n}}{\max\;{\sigma_{e}(k)}}\mspace{14mu}{for}\mspace{14mu} k} \in {\left\lbrack {{k_{p} - \frac{M}{4}},{k_{p} + \frac{M}{4}}} \right\rbrack.}}$

In conventional systems, a transmitter SNDR may be determined using amax σ_(e)(k) determined over the full pulse width. For a node elementconsistent with the present disclosure, e.g., node 102, the SNDR may bedetermined over a portion of the pulse width, as described herein. Forexample, a node element with an SNDR greater than or equal to 22 dB maybe considered conforming. In another example, a node element with anSNDR greater than or equal to 26 dB may be considered conforming. Inanother example, a node element with an SNDR greater than or equal to 27dB may be considered conforming.

COM determination module 146 is configured to determine a channeloperating margin as 20 log₁₀(A_(S)/A_(ni)) where A_(S) corresponds to adetermined signal amplitude and A_(ni) corresponds to a determined noiseamplitude. COM, consistent with the present disclosure, may includeσ_(TX), a Gaussian, signal-dependent noise term that representstransmitter noise and, for a PAM-x transmitted signal with x>2, thesignal degradation factor (i.e., ratio of level mismatch), R_(LM). COMdetermination module 146 may be configured to determine σ_(TX),corresponding to a standard deviation of transmitter noise as seen by areceiver, as:

${\sigma_{TX} = {\sigma_{X}*{A_{S}/10^{\frac{{SNR}_{TX}}{20}}}}},$where A_(S) and σ_(X) represent signal level at the receiver andSNR_(TX) is signal to noise level at the transmitter. In someembodiments, SNR_(TX) may correspond to SNDR. COM may be related to afigure of merit (FOM) that may be determined as:

${{FOM} = {10\;{\log_{10}\left( \frac{\left( {R_{LM}A_{S}} \right)^{2}}{\sigma_{TX}^{2} + \sigma_{J}^{2} + \sigma_{ISI}^{2} + \sigma_{XT}^{2} + \sigma_{N}^{2}} \right)}}},$The channel operating margin (COM) may then be similarly determined byreplacing the denominator of the FOM equation with a noise amplitude(e.g., voltage), A_(ni), that represents a noise and interference levelas:

${{COM} = {20\;{\log_{10}\left( \frac{R_{LM}A_{S}}{A_{ni}} \right)}}},$where A_(ni) is a DER (detector error ratio) quantile (i.e., a signallevel for which the probability of noise exceeding the signal level isas small as a specified DER). For example, DER corresponds to 10⁻⁵ for100GBASE-CR4 and 100GBASE-KR4 physical layers and 3×10⁻⁴ for the100GBASE-KP4 physical layer. A_(ni) may be calculated from a cumulativedistribution function (CDF) of the noise and interference probabilitydistribution, which is calculated using a statistical method from thechannel parameters. The value of σ_(TX) corresponds to the standarddeviation of a Gaussian noise component that is mathematically convolvedwith the distributions of other noise components in the calculation ofA_(ni). σ_(TX) is determined based, at least in part, on SNR_(TX) thatmay correspond to SNDR, thus, σ_(TX) may be based, at least in part, onSNDR.

Thus, transmitter performance measures may include SNDR that may bedetermined utilizing a portion of a pulse width and may include effectsof static level mismatch. Transmitter performance measures (i.e.,characteristics) may further include the signal degradation factor,R_(LM), that is based, at least in part, on static level mismatch. COMmay include the signal degradation factor R_(LM) and at least one termrelated to transmitter noise. To successfully meet an associatedperformance measure, a receiver may thus be configured to tolerate anamount of static level mismatch and transmitter noise. A channel maythen be configured to satisfy a COM target while including static levelmismatch and transmitter noise. Thus, a relatively more accurate systembudget allocation may be satisfied.

FIG. 3 is a flowchart 300 of testing operations according to variousembodiments of the present disclosure. In particular, the flowchart 300illustrates determining performance parameters based, at least in part,on a transmitted test signal. Operations of flowchart 300 may beperformed, for example, by node 102 and/or measurement device 104.Operations of this embodiment include selecting a test signal 302. Forexample, the test signal may be selected from a plurality of possibletest signals, including but not limited to, a single pulse and/or a bitstream modulated by a predefined bit sequence (i.e., a test pattern).Operation 304 includes transmitting the test signal. For example, thetest signal may be generated and transmitted by node 102. Thetransmitted test signal may be captured at operation 306. For example,the test signal may be captured by measurement device 104. Performanceparameter(s) may be determined at operation 308. For example,performance parameters may include SNDR, RMS distortion error, signaldegradation factor (i.e., static level mismatch ratio) and/or COM.Program flow may continue at operation 310.

FIG. 4 is a flowchart 400 of performance parameter determinationoperations according to various embodiments of the present disclosure.In particular, the flowchart 400 illustrates operations configured todetermine normalized effective mid-levels for a PAM-x (x>2) test signal.The operations of flowchart 400 may be performed by, e.g., static leveldetermination module 144. Operations of this embodiment begin withdetermining static levels 402. For example, static levels may bedetermined based, at least in part, on a test pattern configured dwellat each of a plurality of static levels. Operations 404 may includedetermining a minimum eye amplitude (i.e., a static level differential),a level mismatch ratio and normalized effective mid-levels. Program flowmay continue at operation 406.

FIG. 5 is a flowchart 500 of performance parameter determinationoperations according to various embodiments of the present disclosure.In particular, the flowchart 500 illustrates operations configured todetermine a signal to noise and distortion ratio (SNDR). The operationsof flowchart 500 may be performed by, e.g., RMS distortion determinationmodule 142. Operations of this embodiment begin with determining alinear fit to a pulse response 502. For example, the linear fit may bedetermined based on a single pulse. Operations 504 may includedetermining a phase index that corresponds to a pulse response maximum.A standard deviation of the error between a measured pulse and a linearfit to the measured pulse may be determined at operation 506. An RMS(root mean square) distortion error over a portion of the pulse unitinterval may be determined at operation 508. For example, the portionmay be one-half and may be centered at or near the pulse responsemaximum. Operation 510 includes determining a signal to noise anddistortion ratio. Program flow may continue at operation 512.

While the flowcharts of FIGS. 3, 4 and 5 illustrate operations accordingvarious embodiments, it is to be understood that not all of theoperations depicted in FIGS. 3, 4 and/or 5 are necessary for otherembodiments. In addition, it is fully contemplated herein that in otherembodiments of the present disclosure, the operations depicted in FIGS.3, 4 and/or 5, and/or other operations described herein may be combinedin a manner not specifically shown in any of the drawings, and suchembodiments may include less or more operations than are illustrated inFIGS. 3, 4 and/or 5. Thus, claims directed to features and/or operationsthat are not exactly shown in one drawing are deemed within the scopeand content of the present disclosure.

The foregoing provides example system architectures and methodologies,however, modifications to the present disclosure are possible. Forexample, node 102 and/or measurement device 104 may also include chipsetcircuitry. Chipset circuitry may generally include “North Bridge”circuitry (not shown) to control communication between a processor, I/Ocircuitry and memory.

Node 102 and/or measurement device 104 may each further include anoperating system (OS) to manage system resources and control tasks thatare run on each respective device and/or system. For example, the OS maybe implemented using Microsoft Windows, HP-UX, Linux, or UNIX, althoughother operating systems may be used. In some embodiments, the OS may bereplaced by a virtual machine monitor (or hypervisor) which may providea layer of abstraction for underlying hardware to various operatingsystems (virtual machines) running on one or more processing units.

The operating system and/or virtual machine may implement one or moreprotocol stacks. A protocol stack may execute one or more programs toprocess packets. An example of a protocol stack is a TCP/IP (TransportControl Protocol/Internet Protocol) protocol stack comprising one ormore programs for handling (e.g., processing or generating) packets totransmit and/or receive over a network. A protocol stack mayalternatively be comprised on a dedicated sub-system such as, forexample, a TCP offload engine and/or I/O circuitry. The TCP offloadengine circuitry may be configured to provide, for example, packettransport, packet segmentation, packet reassembly, error checking,transmission acknowledgements, transmission retries, etc., without theneed for host CPU and/or software involvement.

Memory 112 and/or memory 132 may comprise one or more of the followingtypes of memory: semiconductor firmware memory, programmable memory,non-volatile memory, read only memory, electrically programmable memory,random access memory, flash memory, magnetic disk memory, and/or opticaldisk memory. Either additionally or alternatively system memory maycomprise other and/or later-developed types of computer-readable memory.

Embodiments of the operations described herein may be implemented in asystem that includes one or more storage devices having stored thereon,individually or in combination, instructions that when executed by oneor more processors perform the methods. The processor may include, forexample, a processing unit and/or programmable circuitry. The storagedevice may include any type of tangible, non-transitory storage device,for example, any type of disk including floppy disks, optical disks,compact disk read-only memories (CD-ROMs), compact disk rewritables(CD-RWs), and magneto-optical disks, semiconductor devices such asread-only memories (ROMs), random access memories (RAMs) such as dynamicand static RAMs, erasable programmable read-only memories (EPROMs),electrically erasable programmable read-only memories (EEPROMs), flashmemories, magnetic or optical cards, or any type of storage devicessuitable for storing electronic instructions.

“Circuitry”, as used in any embodiment herein, may comprise, forexample, singly or in any combination, hardwired circuitry, programmablecircuitry, state machine circuitry, and/or firmware that storesinstructions executed by programmable circuitry. “Module”, as usedherein, may comprise, singly or in any combination circuitry and/or codeand/or instructions sets (e.g., software, firmware, etc.).

In some embodiments, a hardware description language may be used tospecify circuit and/or logic implementation(s) for the various modulesand/or circuitry described herein. For example, in one embodiment thehardware description language may comply or be compatible with a veryhigh speed integrated circuits (VHSIC) hardware description language(VHDL) that may enable semiconductor fabrication of one or more circuitsand/or modules described herein. The VHDL may comply or be compatiblewith IEEE Standard 1076-1987, IEEE Standard 1076.2, IEEE1076.1, IEEEDraft 3.0 of VHDL-2006, IEEE Draft 4.0 of VHDL-2008 and/or otherversions of the IEEE VHDL standards and/or other hardware descriptionstandards.

Thus, consistent with the teachings of the present disclosure, a systemand method are configured to distribute the system budget between atransmitter, a communication channel and a receiver in a communicationsystem. In an embodiment, methods and systems consistent with thepresent disclosure are configured to determine an SNDR based, at leastin part on, a root mean square (RMS) distortion error and to determinethe RMS distortion error over a portion of the pulse response UI thatmay centered at or near a peak of the pulse. Methods and systemsconsistent with the present disclosure are configured to determinestatic levels, a static level mismatch ratio, R_(LM), (i.e., a signaldegradation factor) and to determine a linear fit and the SNDR based, atleast in part, on the determined static levels. Methods and systemsconsistent with the present disclosure are configured to include aGaussian, signal-dependent noise term representing transmitter noise indetermination of COM. For PAM-x modulation, COM may be determined based,at least in part, on the signal degradation factor that is based, atleast in part, on the determined static levels. Thus, the system budgetmay be generally more evenly allocated across system components andtransmitter performance requirements may more accurately reflect actualtransmitter performance characteristics.

Accordingly, the present disclosure provides an example apparatus. Theexample apparatus includes a root mean square (RMS) distortiondetermination module configured to determine an RMS distortion error anda signal to noise and distortion ratio (SNDR), the RMS distortion errordetermined based, at least in part, on a portion of a transmitted pulsecentered at or near a transmitted pulse maximum amplitude and the SNDRdetermined based, at least in part, on the RMS distortion error.

The present disclosure also provides an example system. The examplesystem includes a measurement device comprising a system budget moduleconfigured to select a test signal; a computer node element comprising atransmitter configured to transmit the selected test signal; and a rootmean square (RMS) distortion determination module configured todetermine an RMS distortion error and a signal to noise and distortionratio (SNDR), the RMS distortion error determined based, at least inpart, on a portion of a transmitted pulse centered at or near atransmitted pulse maximum amplitude and the SNDR determined based, atleast in part, on the RMS distortion error.

The present disclosure also provides an example method. The examplemethod includes determining, by a root mean square (RMS) distortiondetermination module, an RMS distortion error based, at least in part,on a portion of a transmitted pulse centered at or near a transmittedpulse maximum amplitude; and determining, by the root mean square (RMS)distortion determination module, a signal to noise and distortion ratio(SNDR) based, at least in part, on the RMS distortion error.

The present disclosure also provides an example system that includes oneor more storage devices having stored thereon, individually or incombination, instructions that when executed by one or more processorsresult in the following operations including: determining an RMSdistortion error based, at least in part, on a portion of a transmittedpulse centered at or near a transmitted pulse maximum amplitude; anddetermining a signal to noise and distortion ratio (SNDR) based, atleast in part, on the RMS distortion error.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described (or portions thereof), and it isrecognized that various modifications are possible within the scope ofthe claims. Accordingly, the claims are intended to cover all suchequivalents.

Various features, aspects, and embodiments have been described herein.The features, aspects, and embodiments are susceptible to combinationwith one another as well as to variation and modification, as will beunderstood by those having skill in the art. The present disclosureshould, therefore, be considered to encompass such combinations,variations, and modifications.

What is claimed is:
 1. A system to determine transmitter noise, thesystem comprising: network interface circuitry; and measurementcircuitry couple to the network interface circuitry, the measurementcircuitry to: select at least one signal; transmit the selected at leastone signal across the network interface circuitry; capture at least aportion of the transmitted at least one signal; determine at least onevalue indicative of a root-mean-square (RMS) distortion error present inthe captured portion of the transmitted at least one signal; anddetermine at least one value indicative of a signal to noise anddistortion ratio (SNDR) based, at least in part, on the determined atleast one value indicative of the RMS distortion error.
 2. Thetransmitter noise determination system of claim 1 wherein the at leastone signal comprises at least one defined test signal.
 3. Thetransmitter noise determination system of claim 2 wherein the at leastone defined test signal includes at least one pulse.
 4. The transmitternoise determination system of claim 3 wherein the at least one definedtest signal includes at least a four-level pulse amplitude modulated(PAM-4) test signal.
 5. The transmitter noise determination system ofany of claim 3 or 4 wherein the measurement circuitry to determine atleast one value indicative of an RMS distortion present in the capturedportion of the transmitted at least one signal comprises: measurementcircuitry to determine at least one value indicative of an RMSdistortion present in the captured portion of the at least one pulsecentered at about a maximum amplitude of the at least one pulse.
 6. Thetransmitter noise determination system of claim 1, the measurementcircuitry to further: determine at least one value representative of aminimum static level differential; determine at least one valuerepresentative of a level mismatch ratio; and determine at least onevalue representative of at least one normalized effective mid-level;wherein the at least one value indicative of the SNDR is based, at leastin part on the at least one value representative of the minimum staticlevel differential.
 7. A transmitter noise measurement apparatuscomprising: at least one measurement circuit coupleable to a networkinterface, the at least one measurement circuit to: select at least onetest signal that includes a pulse amplitude modulated test signal thatincludes a plurality of pulse amplitudes (PAM-x); transmit the selectedat least one signal across the network interface circuitry; capture atleast a portion of the transmitted at least one signal; determine atleast one value indicative of a root-mean-square (RMS) distortion errorpresent in the captured portion of the transmitted at least one signal;and determine at least one value indicative of a signal to noise anddistortion ratio (SNDR) based, at least in part, on the determined atleast one value indicative of the RMS distortion error.
 8. Thetransmitter noise measurement apparatus of claim 7 wherein the at leastone measurement circuit to select at least one test signal that includesa pulse amplitude modulated test signal that includes a plurality ofpulse amplitudes (PAM-x) comprises: at least one measurement circuit toselect at least one test signal that includes a pulse amplitudemodulated test signal that includes a plurality of pulse amplitudes(PAM-x) that produce a defined bit sequence.
 9. The transmitter noisemeasurement apparatus of claim 7 wherein the at least one measurementcircuit to select at least one test signal that includes a pulseamplitude modulated test signal that includes a plurality of pulseamplitudes (PAM-x) comprises: at least one measurement circuit to selectat least one test signal that includes a pulse amplitude modulated testsignal that includes a plurality of pulse amplitudes (PAM-x) thatincludes uniform level spacing in which each pulse amplitude isequidistant from each adjacent pulse amplitude.
 10. The transmitternoise measurement apparatus of claim 7 wherein the at least onemeasurement circuit to select at least one test signal that includes apulse amplitude modulated test signal that includes a plurality of pulseamplitudes (PAM-x) comprises: at least one measurement circuit to selectat least one test signal that includes a pulse amplitude modulated testsignal that includes a plurality of pulse amplitudes (PAM-x) thatincludes variable level spacing in which each pulse amplitudecorresponds to at least one measurement circuit performancecharacteristic.
 11. The transmitter noise measurement apparatus of claim7, the at least one measurement circuit to further: determine at leastone value indicative of a minimum eye amplitude (S_(min)); determine atleast one value indicative of a level mismatch ratio; determine at leastone value indicative of a direct current (DC) offset; and determine atleast one value indicative of at least one normalized effectivemid-level voltage.
 12. A method to determine transmitter noise, themethod comprising: selecting, by measurement circuitry, at least onesignal; transmitting, by the measurement circuitry, the selected atleast one signal across a communicably coupled network interfacecircuitry; capturing, by the measurement circuitry, at least a portionof the transmitted at least one signal; determining, by the measurementcircuitry, at least one value indicative of a root-mean-square (RMS)distortion error present in the captured portion of the transmitted atleast one signal; and determining, by the measurement circuitry, atleast one value indicative of a signal to noise and distortion ratio(SNDR) based, at least in part, on the determined at least one valueindicative of the RMS distortion error.
 13. The transmitter noisedetermination method of claim 12 wherein selecting at least one signalcomprises: selecting, by the measurement circuitry, at least one definedtest signal.
 14. The transmitter noise determination method of claim 13wherein selecting at least one defined test signal comprises: selecting,by the measurement circuit, at least one defined test signal thatincludes at least one pulse.
 15. The transmitter noise determinationmethod of claim 14 wherein selecting at least one defined test signalthat includes at least one pulse comprises: selecting, by themeasurement circuit, at least one defined test signal that includes atleast a four-level pulse amplitude modulated (PAM-4) test signal. 16.The transmitter noise determination method of any of claim 14 or 15wherein determining at least one value indicative of an RMS distortionpresent in the captured portion of the transmitted at least one signalcomprises: determining, by the measurement circuitry, at least one valueindicative of an RMS distortion present in the captured portion of theat least one pulse centered at about a maximum amplitude of the at leastone pulse.
 17. The transmitter noise determination method of claim 12,further comprising: determining, by the measurement circuitry, at leastone value representative of a level mismatch ratio; determining, by themeasurement circuitry, at least one value representative of at least onenormalized effective mid-level voltage; and determining, by themeasurement circuitry, at least one value representative of a minimumstatic level differential, wherein the at least one value indicative ofthe SNDR is based, at least in part on the at least one valuerepresentative of the minimum static level differential.
 18. A storagedevice that includes machine-readable instructions that, when executedby a measurement circuit, cause the measurement circuit to providetransmitter noise determination measurement circuity that: selects atleast one signal; causes a communicably coupled network interface totransmit the selected at least one signal; captures at least a portionof the transmitted at least one signal; determines at least one valueindicative of a root-mean-square (RMS) distortion error present in thecaptured portion of the transmitted at least one signal; and determinesat least one value indicative of a signal to noise and distortion ratio(SNDR) based, at least in part, on the determined at least one valueindicative of the RMS distortion error.
 19. The storage device of claim18 wherein the machine-readable instructions that cause the transmitternoise determination measurement circuity to select at least one signalfurther cause the transmitter noise determination measurement circuityto: select at least one defined test signal.
 20. The storage device ofclaim 19 wherein the machine-readable instructions that cause thetransmitter noise determination measurement circuity to select at leastone defined test signal further cause the transmitter noisedetermination measurement circuity to: select at least one defined testsignal that includes at least one pulse.
 21. The storage device of claim20 wherein the machine-readable instructions that cause the transmitternoise determination measurement circuity to select at least one definedtest signal that includes at least one pulse further cause thetransmitter noise determination measurement circuity to: select at leastone defined test signal that includes at least a four-level pulseamplitude modulated (PAM-4) test signal.
 22. The storage device of anyof claim 20 or 21 wherein the machine-readable instructions that causethe transmitter noise determination measurement circuity to determine atleast one value indicative of an RMS distortion present in the capturedportion of the transmitted at least one signal further cause thetransmitter noise determination measurement circuity to: determine atleast one value indicative of an RMS distortion present in the capturedportion of the at least one pulse centered at about a maximum amplitudeof the at least one pulse.
 23. The storage device of claim 18 whereinthe machine-readable instructions further cause the transmitter noisedetermination measurement circuity to: determine at least one valuerepresentative of a level mismatch ratio; determine at least one valuerepresentative of at least one normalized effective mid-level voltage;and determine at least one value representative of a minimum staticlevel differential, wherein the at least one value indicative of theSNDR is based, at least in part on the determined at least one valuerepresentative at least one value representative of the minimum staticlevel differential.
 24. A transmitter noise determination system,comprising: at least one network interface; and at least one measurementcircuit communicably coupled to the at least one network interface, theat least one measurement circuit to: select a test signal, the testsignal including a pulse amplitude modulated test signal that includesat least four modulation levels; measure a pulse response based on thereceived test signal; determine a linear fit to the measured pulseresponse; determine a standard deviation of an error (σ_(e)) between themeasured pulse response and the linear fit to the measured pulseresponse; and determine a signal-to-noise-and-distortion ratio (SNDR)that is base-10 logarithmically proportional to the inverse of thedetermined standard deviation of the error between a measured pulseresponse and the linear fit to the measured pulse response.
 25. Thesystem of claim 24 wherein the pulse amplitude modulated test signalconsists of four different voltage amplitudes (V_(A), V_(B), V_(C),V_(D)).
 26. The system of claim 25 wherein the pulse amplitude modulatedtest signal consists of a repeated two-bit sequence.
 27. The system ofclaim 25, further comprising a static determination module, the staticdetermination module to determine a minimum static level differentialaccording to the following:$S_{m\; i\; n} = {\frac{\min\left( {{V_{D} - V_{C}},{V_{C} - V_{B}},{V_{B} - V_{A}}} \right)}{2}.}$28. The system of claim 25, further comprising a static determinationmodule, the static determination module to determine a DC offsetaccording to the following:$V_{avg} = {\frac{V_{A} + V_{B} + V_{C} + V_{D}}{4}.}$
 29. The system ofclaim 28, further comprising a static determination module, the staticdetermination module to determine a first normalized effective mid-levelvoltage according to the following:$V_{1} = {\frac{V_{B} - V_{avg}}{V_{A} - V_{avg}}.}$
 30. The system ofclaim 28, further comprising a static determination module, the staticdetermination module to determine a second normalized effectivemid-level voltage according to the following:$V_{2} = {\frac{V_{C} - V_{avg}}{V_{D} - V_{avg}}.}$
 31. The system ofclaim 29, further comprising a static determination module, the staticdetermination module to determine a level mismatch ratio according tothe following: $R_{L\; M} = {\frac{6*S_{m\; i\; n}}{V_{D} - V_{A}}.}$